We all know that kv defines how fast a motor spins in relation to the voltage, or RPM = kv * v. If so, why are two 2204 2300kv motors from different brands not the same? Why is a cobra 2204 2300kv diferent from an Emax 2204 2300kv and Sunnysky 2204 2300kv?
The reason lais in the RPM formula:
RPM = Kv * V
If we try comparing two 2204 2300kv motors using this RPM formula we will find that they are identical, since they are both marked as 2300kv. But, this formula does not describe the “whole story”. V refers to a voltage slightly different than your battery’s voltage.
This is what V really is:
V = Vbatt – Vloss
RPM = Kv * (Vbatt – Vloss)
Vbatt is your battery voltage (either 12.6v on 3s or 16.8v on 4s). Vloss is voltage “lost” because of “something” (we’ll get there).
This “something” wastes part of the energy sent to the motor and results in a slower RPM than the one we “anticipated” before this post 🙂
This wastful “something” is internal resistance – the motor’s internal resistance to be exact, or Rm.
Vloss = Rm * I
RPM = Kv * [Vbatt – (Rm*I)]
If so, what is Rm?
This variable is listed by most manufacturers. Either by stating an internal resistance or a no load amperage.
These are a factor of how the coils are made. What are they made of, how thick the wire is, heat dissipation and more.
Using the no load amperage we can calculate the internal resistance of a motor:
Rm = Vbatt / Istall
The internal resistance of a motor has a negative effect on the motor speed. This effect is the reason why motor speeds vary from one brand to the other.